1. Field of the Invention
The present invention relates to a chromatic aberration corrector for correcting the electron optical system of an electron microscope for chromatic aberration and also to a method of controlling the chromatic aberration corrector.
2. Description of Related Art
In the past, a method using multipole lenses for producing quadrupole fields to correct the electron optical system (such as an objective lens) of an electron microscope for chromatic aberration has been known. The characteristics of quadrupole fields have been already investigated in detail, and the orbit of electrons through a quadrupole field and aberrations in the field have been clarified (see P. W. Haukes, Quadrupoles in Electron Lens Design (Advance in Electronics & Electron Physics, Supplement 7), Academic Press (1970), New York and London). With a view to eliminating chromatic and spherical aberrations in a magnetic field used in an electron microscope, combinations of plural quadrupole fields have been studied. As a result, the following conclusion concerning the number of combined quadrupole fields has been derived: In an aberration corrector using quadrupole fields, the focal plane in the x-orbit and the focal plane in the y-orbit must be the same and the magnification in the x-direction must be identical with the magnification in the y-direction, i.e., the so-called stigmatic conditions must hold. Any combination of two quadrupole fields cannot satisfy these conditions (see, for example, P. W. Haukes, Quadrupoles in Electron Lens Design (Advance in Electronics & Electron Physics, Supplement 7), Academic Press (1970), New York and London, p. 90). With two stages of quadrupole fields, it is possible to bring the focal plane of the x-orbit and the focal plane of the y-orbit into coincidence. However, it is substantially impossible to make the magnifications in the x- and y-directions the same. This is referred to as pseudo-stigmatic conditions (see, for example, Katsumi Ura, “Electron and Ion Beam Optics (in Japanese)”, Kyoritsu Publishing Company (1994), first ed., chapter “Quadrupole Lenses”). Therefore, regarding aberration correctors using quadrupoles, combinations of four or more stages of quadrupoles that satisfy the stigmatic conditions have been studied and commercialized.
In the conventional chromatic aberration correction using four stages of quadrupoles, correction in the x-direction and correction in the y-direction are performed using different stages of quadrupole fields because the characteristics of a quadrupole field are directly employed for chromatic aberration correction. That is, in the x- and y-orbits, if convergence is achieved in one of the orbits, divergence occurs in the other. If the correction plane is different between the x- and y-directions, it is impossible to bring their correction planes simultaneously into coincidence with the objective lens plane in which chromatic aberration is introduced. Where the chromatic aberration correction plane and introduction plane are not coincident, collateral aberrations proportional to the distance between them are produced.